Treatment of polyhydric solutions



Nov. 11, 1958 N. w. ROSENBERG 2,860,091

TREATMENT OF POLYHYDRIC SOLUTIONS Filed Sept. 22, 1955 SPECIFICRESISTANCE OF CANE JUICE DlLUTED TO VARIOUS BRIX LEVELS BY WATERADDITION, OR

SPECIFIC RESISTANCE A" C) SOLUTION TEMPERATURE 25's O SOLUTIONTEMPERATURE 40C I A SOLUTION TEMPERATURE 60 5 50 l I I I0 so so 10 BRIXM3 Nbflm (8009226919,

2,860,091 TREATMENT OF POLYHYDRIC SOLUTIONS Norman W. Rosenberg, NewtonCenter, Mass, assignor to Ionics, Incorporated, Cambridge, Mass, incorporation of Massachusetts Application September 22, 1955, Serial No.535,804

7 Claims. (Cl. 204-138) This invention relates to processes forelectrically demineralizing normally viscous aqueous liquids such assyrups, molasses, glycerine, aqueous mannitol and sorbitol and otheraqueous solutions of unionized polyhydric materials and may be appliedinter alia to sacchariferous solutions of e. g. saccharose, levulose,sorbose, mannose, maltose, fructose, dextrose, molasses of the canesugar and beet sugar industry, starch hydrolysates, malt extracts, woodsugar solutions, etc. The invention here disclosed makes use of subjectmatter disclosed in copending U. S. patent applications Serial Nos.300,302 to Katz and Rosenberg and 329,639 to McRae, now Patent No.2,777,811, filed July 22, 1952, and January 5, 1953, respectively; andU. S. Patent Nos. 2,708,658 and 2,694,680 issued July 18, 1952, andNovember 13, 1954, respectively. V

In its usual design, a multi-chamber demineralizing apparatus consistsof a large number (perhaps several hundred) alternately-spaced anion andcation permselective membranes with spacer members therebetween. Thespacer members usually have tortuous paths which, when in face-to-facecontact with the membranes, vformthe concentrating and diluting chambersof the demineralizer. The end chambers of the apparatus contain theelectrodes for passing a direct current across all the chambers. Theprocess streams and electrode wash streams are passed through theirrespective chambers from manifolds and are controlled as to rate offlow, pressure, temperature, etc. Such apparatus, permselectivemembranes, and modes of operation are disclosed in applicants copendingapplication Serial No. 428,072, filed April 29, 1954. j

It is well known that in the processing of sugar beets, sugar cane, andother saccharides, the presence of organic and inorganic saltsin theprocess sugar solutions interferes with the recovery of sugar. Toovercome these difiiculties, it has heretofore been proposed to deionizethe sacchariferous process solutions by passing 'them throughion-exchange resin beds. Objections to this procedure include thefollowing:

(a) Owing to the necessity for regenerating and washing, dilute sugarsolutions are produced which, for economic reasons, must be recovered.The cost of recovery in many cases oifsets any gains from the'exchangetreatment.

(b) The high cost of chemical regeneration for solutions containingappreciable quantities of salts.

(c) The production of monosaccharides from polyor di-saccharides by thecatalytic atcion of the acid-form cation exchangers. In general, the'monosaccharides themselves inhibit the crystallization of higher sugarsand constitute in addition a direct loss of the higher sugars. Forexample, dextrose and fructose resulting from acid catalysis of sucroseinhibit the recovery of sucrose. It is clear that there is no gain inremoving one inhibitor and adding another.

(d) The accidental leakage of acid (or alkali) into the treated sugarsolution owing to insuflicient rinsing, channeling of, rinse, etc. i i ii 2,860,091 Patented Nov. 11,

(e) The high molecular weight charged impurities usually of naturalorigin, present in such solutions are irreversibly bound by theexchangers, particularly the anion exchangers (which is unfortunatelymuch more expensive than the cation exchanger), and are removed if atall only with considerable effort and expense. This effect is evidencedby a rapid loss of capacity of'the exchangers and an increasingconsumption of chemicals required for regeneration.

For these and other reasons the ion-exchange treatment of sugar andother polyhydric materials has not been widely accepted.

It is proposed herein to demineralize electrically saidpolyhydricsolution's by means of a multi-membrane systern of thecharacter noted above. 7

It is apparent that many of the problems encountered in the granularion-exchange deionization of polyhydric solutions occur in altered formin membrane demineralization. Among these are (a) The diffusion of thepolyhydric ion-electrolyte from the dilute to the concentrate chambersthrough the membranes. The diffusion constitutes a loss of sugar whichmay offset any gains resulting from demineralization. This problem hasbeen solved by operating at high current densities so as to reduce to aminimum the time available for difiusion.

(b) The irreversible absorption of charged polymeric matter on themembranes resulting in increased resistance to the passage of currentand therefore in increased energy consumption.

are employed, the resistance of the demineralizer increasing rapidly andsteadily to impractical values. Polarizing current densities arediscussed more fully below and in the above-cited Rosenberg application.

(c) The tendency for bubbles to form or lodge in the dilute chambers isgreat. These restrict the hydraulic flow and are not easily dislodgedresulting in rapid depletion' of conducting species in the stream andleading thereby to anomalous polarization, i.' e., polarization whichwould not be expected on the basis of'the average current density andaverage concentration in'the demineralizer. This difficulty has beenovercome by main' taining the flow in the dilute chambers at an apparentsuperficial linear velocity in excessof 5 centimeters per second whichseemingly is sufiicient to dislodge any bub bles which-may form. 0

Superficial velocity as used in this application refers to the ratio ofthe volumetric rate of flow to the cross sectional area available forflow. For example, if the tortuous path in a spacer member has a widthofl cm. and a thickness of 0.1 cm. and theflow rate is 2.5 be. persecond, then the superficial velocity is 25 cm. persecond'.

(d) Concentrated aqueous solutions of polyhydric materials which containa minor component of electrolytes have a low equivalent conductanceowing primarily to the high viscosity. This phenomena operates againstthe process in two ways.' First, the electrical resistance is so muchgreater than for a water solution having the same concentration ofelectrolyte, that the electrical en ergy consumption is greatlyincreased. (The viscosity of a 60 Brix solution is about fifty timesthat of water at room temperature.) Secondly, the maximum ionpolarizingcurrent density is reduced to the same extent as the equivalentconductance tending toward specific electrolyte transfer rates which areso low as to make the investment cost impractically high. Theseobjections have been overcome as will be more formally set forth belowby operating in the preferred form at concentra tions in the range of 20to 40 Brix and at temperature up to 60 C. or even higher.

(e) The cost of pumping the normally viscous solu It has been found thatthis effect becomes most profound when polarizing current densitiestions is excessively high. This has been overcome by operating attemperatures up to 60 C. or even up to 90 C.

It is apparent that for economy of capital investment, the highestpossible current-density is desired. The latter corresponds to thehighest electrolyte transfer rate which may be expressed, for-example,as pounds of ash per hour per square foot of membrane. However, at eachoperating condition (with regard to flow rate, temperature, electrolyteand non-electrolyte concentration, etc.)

there is a current density above which the electrical resistance of thedemineralizer becomes abnormally high, and current efiiciencies fallsharply. This value is referred to herein as the polarizing currentdensity. It is believed that at this value the solution region of themembrane-solution interface in the diluting chambers becomes depleted inhighly dissociated and therefore highly con ductive electrolyte specieand, thereafter conduction de pends on the dissociation of water andweakly conductive electrolytes. The latter may comprise large ormultiplecharged ions (such as proteins and carboxylic acids resultingfrom the oxidation of saccharides) which may be more or lessirreversibly held by the membranes resulting in abnormally high membraneresistance. High molecular weight weakly conducting materials in canesugar molasses are of this type. Accordingly, the polarizing currentdensity should be approached but not exceeded for eificient and economicoperation. When polarizing current densities are exceeded, operationalinstability usually results, evidenced by an increase, with time, ofelectrical resistance and an accompanying decrease of currentefiiciency. This effect is not easily reversible, and a return tonon-polarizing current densities may show a more or less permanentincrease in membrane resistance.

The onset of polarizing current densities is experimentally found todepend on solution conductance which is in turn controlled by theconcentration of both electrolytes and non-electrolytes and by solutionviscosity. Four generalizations can be made experimentally. (1) At aconstant superficial solution velocity the polarizing current density isapproximately proportional to solution conductivity; (2) for a givensolution the polarizing current density is approximately proportional tothe square root of the superficial velocity, which should be in excessof cm. per second. It has been found that the current density to benon-polarizing should always be less than about /2 the product of thespecific conductance of the solution in the diluting chambers and thesquare root of the superficial velocity of the solution in the dilutingchambers, the current density expressed in amperes per squarecentimeter, the specific conductance in amperes pervolt per cubiccentimeter and the velocity in centimeters per second; (3)demineralization of the solution passing through the dilute chambersshould be less than 90% in any one passage to avoid localizedpolarization and instability; (4) the optimum temperature fordemineralization is about 60 C. Above 60 C. permselectivemembranesbecome increasingly unstable, while below 60C. the resistance increasesslowly. While 60 C. is optimum for most purposes, some successfuloperation may also be found in the range up to about 90 C.

Any method of operation which lowers the energy consumption at theexpense of a proportional increase in the plant size is not usuallyeconomically sound. While the de-ashing of most sugar solutions at a lowcurrent density may require an extremely low energy input, the plantinvestment in most cases is so large as to make the processuneconomical. In order to render the process practical, it is necessaryto use the maximum permissible currents. It will therefore be apparentthat the conductance of the feed stream is an important factor in thedemineralization of sugar solutions.

It has been found that for a given ratio of polyhydric non-electrolyte(e. g., sucrose) to electrolyte (e. g., ash) in aqueous solution thatthe maximum specific conduct- 4 ance (i. e., minimum specificresistance) occurs at a total concentration between about 20 and about40 Brix and usually about 30 Brix. This relationship is clearly shown inthe figure, where it can be seen that the minimum resistance on dilutionof a sugar solution (corresponding to the maximum conductance) is foundat about 30 Brix and that a range of from about 20 to about 40 Brixgives conductances which are not substantially different from that at 30Brix, thus allowing the highest nonpolarizing ash transfer to beemployed in this range.

It is also apparent from the figure that, as the temperature increasesto about 60 C., the conductivity becomes increasingly favorable, and ithas been experimentally shown to permit an increase in maximumnon-polarizing current density of about 50% over the maximum at roomtemperature (15 to 25 C.). It was also experimentally found that therate of inversion of diand poly-saccharides is not appreciable in thetime required for demineralization up to about C. at an essentiallyneutral demineralization and that the thermal stability of the.permselective membranes is adequate.

It is apparent that in accordance With the present invention theconcentration of the aqueous polyhydric solution to be demineralizedshould be controlled by the addition or removal (e. g., by evaporation)of water or other aqueous polyhydric solutions of difleringconcentration so that a Brix of from about 20 to 40, or preferably about30, is obtained. This will result in the most economical and efiicientdemineralization of the solution. However, some results would also beobtained at a Brix of about 10 up to about 45 at temperatures up to C.

The following examples are given to illustrate the invention wherecontrol of solution concentration and temperature of electrodialysis aredemonstrated.

EXAMPLE 1' Hawaiian cane juice A multi-membrane demineralizer unit shownin Fig. 1 of the Rosenberg application Ser. No. 428,072, notedhereinabove was constructed for use with various sugar solutions. AHawaiian cane juice had the following analysis: sucrose 12%, invert0.6%, sulfated ash 0.9%, organic non-sugars 1.0%, and water 85.5%. Thecation membranes were based on sulfonate groups and hada resistance of15 ohms per square centimeter. The anion membranes had a resistance of25 ohms per square centimeter and was based on quarternary ammoniumgroups. These permselective ion exchange membranes and the method ofmaking the same are clearly and specifically set forth in theapplication to Katz and Rosenberg, Ser. No. 300,302, noted above. It wasfound that by operating with this cane juice at 15 Brix and asuperficial velocity of 45 cm./sec., a current density of 11 ma./cm. wasobtainable with no operational, difficulty. On the other hand, increaseof the current density to 15 ma./cm. resulted in an unstable resistancewhich necessitated a voltage increase with time rising from initially1.4 volts per cell pair to 3.5 volts per cell pair after 24 hours, atwhich time it was still rising. As opposed to this operation, the 11ma./cm. required 1.0 volt per cell pair initially and after 24 hoursstill required only 1.0 volt per cell pair.

Water was evaporated from another sample of this juice until aconcentration of 30 Brix was obtained, and under these conditions aninitial current density of 23 mat/cm. was obtained at 1.2 volts per cellpair and remained stable over a 24-hour period. On the other hand,operation at 30 ma./cm. caused a rapid rise in the voltage required tomaintain this current density. Athird sample of the Hawaiian cane juicewas evaporated to a concentration of 50 Brix. In this case 1.4 volts percell pair passed 12 ma./cm. and this remained constant. On the otherhand, increase of the current density to 14 ma./cm. resulted in atripling of the applied voltage over a period of 24 hours. In otherwords, the maximum current and salt removal was obtained at 30 Brix instable operation, and either increase or decrease of this Brix resultedin a lower stable (i. e., non-polarizing) current density.

EXAMPLE 2 Louisiana blackstrap molasses A sample of Louisiana blackstrapmolasses of 80 Brix was treated in the membrane demineralizer ofExample 1. The analysis was as follows: 36% sucrose, 32% invert, 8%sulfated ash, 6% organic non-sugars, and 18% water.

Addition of water to reduce the Brix to 40 indicated, at 20 cm./sec.flow, a stable current density of 40 ma./cm. with no voltage change overa 24-hour period. A current density of 60 ma./cm. was definitelynonstable in this system, and a voltage rise from 1.2 to 5.0 volts percell pair resulted in 24 hours.

On the other hand, reduction of the Brix to 28 resulted in a stablecurrent density of 90 ma./crn. at a voltage of 1.0 volt per cell pair.Reduction of the Brix by further addition of water to 12 Brix resultedin a decrease in the stable current density to 30 ma./cm. and it wasimpossible to obtain stable operation at 40 ma./cm. It will be seen fromthe analysis that this polyhydric solution has a significantly differentcomposition from that of Example 1, but the same general efiect wasfound, namely, that the highest salt transfer per unit area per per unittime was obtained when a concentration of approximately 30 Brix wasachieved. Note that in this example this concentration was effected bydilution, whereas for the cane juice of Example 1, it was obtained byconcentration.

EXAMPLE 3 Glycerine A process stream in the glycerine industry containsapproximately 80% glycerine, 8% NaCl, and 12% water.

It was found that the maximum stable current density which could beachieved in the unit of Example 1 using the undiluted material wasma./cm. and at higher currents the unit resistance rose rapidly withtime. On the other hand, addition of water to produce a solution 30% inglycerine, 3% in NaCl, and 67% in water (corresponding to about 33 Brix)at the same temperature and linear velocity resulted in a currentdensity of 40 ma./cm. with no deleterious effects. Dilution of theoriginal glycerine solution to 10% glycerine, 1% NaCl, and 89% waterresulted in a stable current density of only 20 ma./cm. and it was foundthat at 30 ma./cm. rapid rise of resistance with time was obtained.

EXAMPLE 4 Dextrose In the dextrose industry a process stream is obtainedcontaining as major constituents 20% dextrose and 0.03 N H SO It is, ofcourse, desirable to reduce this acidity to allow sale of anon-acid-containing product. Treatment of this solution in membranedemineralizers at room temperature was found to result in a permissiblecurrent density of 6 ma./cm. On the other hand, increase in temperatureto 60 C. resulted in a stable current of 12 ma./cm. and thus twice theacid transfer was achievable No deleterious effects of heat on themembranes were found. While higher temperatures permit higher stable(nonpolarizing current densities), the ion-exchange groupings of thepermselective membranes become increasingly unstable, and the pumps,piping, heat exchangers, etc., which carry the raw acid dextrosesolution, are subject to corrosion at increasingly impractical rates.The latter problem may be solved only with heavy and undesirable capitalexpenditures.

Having thus disclosed our invention, I claim as new and desire to secureby Letters Patent:

1. In the demineralization of sacchariferous solutions in amulti-chamber electrodialysis unit wherein the chambers thereof areseparated by permselective ion-exchange membranes, said solutions havingminor components of electrolytes to be removed therefrom, the steps ofadjusting said solutions to a concentration in the range of about 20 toabout 40 Brix and to a temperature below about 60 C., passing saidsolutions at a velocity in excess of 5 centimeters per second throughthe diluting chambers of said multichamber electrodialysis unit, andpassing a direct electric current through said unit.

2. In the demineralization of glycerine solution in a multichamberelectrodialysis unit wherein the chambers thereof are separated bypermselective ion-exchange membranes, said solution having minorcomponents of electrolytes to be removed therefrom, the steps ofadjusting said solution to a concentration in the range of about 20 toabout 40 Brix and to a temperature below about 60 C., passing saidsolution at a velocity in excess of 5 centimeters per second through thediluting chambers of said multichamber electrodialysis unit, and passinga direct electric current through said unit. I

3. In the demineralization of sacchariferous solutions in a multichamberelectrodialysis unit wherein the chambers thereof are separated bypermselective ion-exchange membranes, said solutions having a Brixgreater than 40 with minor components of electrolytes to be removedtherefrom, the steps of diluting said solutions to a range of about 20to about 40 Brix, passing said solutions through the diluting chambersof said multichamber electrodialysis unit, and passing a direct electriccurrent through said unit.

4. In the demineralization of sacchariferous solutions in a multichamberelectrodialysis unit wherein the chambers thereof are separated bypermselective ion-exchange membranes, said solutions having a Brix lessthan 20 with minor components of electrolytes to be removed therefrom,the steps of concentrating said solutions to a range of about 20 toabout 40 Brix, passing said solutions through the diluting chambers ofsaid multichamber electrodialysis unit, and passing a direct electriccurrent through said unit.

5. The method of claim 1 wherein the solution is adjusted to aconcentration of about 30 Brix.

6. In the demineralization of an aqueous solution of organic polyhydricnonelectrolyte in a multichamber electrodialysis unit wherein thechambers thereof are separated by permselective ion-exchange membranes,said solution having minor components of electrolytes to be removedtherefrom, the steps of passing said solution at a velocity in excess of5 centimeters per second at a tem perature in the range of from roomtemperature to about less than centigrade and at a concentration in therange of about 20 to 40 Brix through the diluting chambers of saidmultichamber electrodialysis unit, and passing a direct electric currentthrough said unit.

7. The method of claim 6 wherein the solution is adjusted to aconcentrate of about 30 Brix.

References Cited in the file of this patent UNITED STATES PATENTS751,179 Kollrepp et a1 Feb. 2, 1904 1,577,669 Wolf et a1. Mar. 23, 19261,972,561 Heubaum Sept. 4, 1934 2,671,055 Aten et a1. Mar. 2, 19542,694,680 Katz et a1. Nov. 16, 1954 2,708,658 Rosenberg May 17, 1955

1. IN THE DEMINERALIZATION OF SACCHARIFEROUS SOLUTIONS IN AMULTI-CHAMBER ELECTRODIALYSIS UNIT WHEREIN THE CHAMBERS THEREOF ARESEPARATED BY PERMSELECTIVE ION-EXCHANGE MEMBRANES, SAID SOLUTIONS HAVINGMINOR COMPONENTS OF ELECTROLYTES TO BE REMOVED THEREFROM, THE STEPS OFADJUSTING SAID SOLUTIONS TO A CONCENTRATION IN THE RANGE OF ABOUT 20* TOABOUT 40* BRIX AND TO A TEMPERATURE BELOW ABOUT 60*C., PASSING SAIDSOLUTIONS AT A VELOCITY IN EXCESS OF 5 CENTIMETERS PER SECOND THROUGHTHE DILUTING CHAMBERS OF SAID MULTICHAMBER ELECTRODIALYSIS UNIT, ANDPASSING A DIRECT ELECTRIC CURRENT THROUGH SAID UNIT.